The invention concerns the use of tires equipped with safety insert and, in particular, the detection of bearing of the tire on the safety insert. It proposes an insert which warns the driver as soon as the tire bears on it after a flat or in case of substantial pressure loss.
These safety inserts are, in general, mounted on the rim inside the tire. Their function is to take up the load in case of tire failure.
The bearing of the tire on the safety insert is accompanied by a more or less marked degradation of its performance which may not be perceptible to the driver through the behavior and comfort of the vehicle. Furthermore, the operating lifetime of these inserts is limited. It is therefore essential for safety that the driver be warned as soon as a tire bears on its safety insert, so that the driver can follow the manufacturer""s instructions.
Several safety inserts incorporating means for warning the driver of such bearing have previously been proposed.
U.S. Pat. No. 4,262,724 proposes a safety insert intended to be mounted in an assembly comprising a tire and a rim and radially outward from the rim. This insert has a radially outer surface which defines a radial support for the crown of the tire when the tire is deflated, as well as means for generating vibrating warning signals on a run-flat condition. These means are a variation of the run-flat radius of the insert between a minimum radius and a maximum radius or one or more bulges.
These solutions pose several problems. In order to be detected by the driver, the amplitude of the variations has to be high and that causes marked discomfort for the passengers of the vehicle, at least at certain speeds. On the other hand, it can greatly degrade the behavior of the tire and rim assembly concerned, particularly on acceleration and braking. All one can do is thus to avoid instant stopping of the vehicle in case of tire failure. On the other hand, when it is desired to use the vehicle, even at limited speed, over long distances, the warning transmitted by the insert has to be compatible with driving safety, not impairing the mechanical operation, while remaining perfectly perceptible to the driver either directly or by means of a suitable detection device.
Furthermore, patent application WO 94/03338 proposes a system for detecting the bearing of a tire on a safety insert. The system comprises a wheel accelerometer, placed on one of the wheel suspensions and measuring vertical accelerations. The accelerometers are connected to a central processing unit. The analysis is based on the detection on bearing of a resonance mode characteristic of a run-flat condition.
The invention concerns a safety insert which warns the driver, either directly or indirectly, upon bearing of the tire against the safety insert over a very wide range of speeds of the vehicle, while retaining a character tolerable to both the driver and the mechanics in the range of authorized speeds.
xe2x80x9cVehiclexe2x80x9d is understood below to mean an autonomous running unit, passenger car, tractor or trailer, truck, motorcycle, etc.
A first safety insert according to the invention, intended to be mounted in an assembly comprising a tire and a rim of a vehicle and radially outward from the rim, has a radially outer bearing surface which defines a radial support for the crown of the tire when said tire is deflated and means for generating vibrating warning signals on a run-flat condition. This insert is characterized in that said means are a variation of a characteristic chosen from the group of outer radius and radial stiffness of said insert as a function of azimuth xcex1, which presents at least four maxima distributed in at least twice two different values M1 and M2.
In its simplest embodiment, characteristic M presents as a function of azimuth xcex1 two maxima M1 diametrically opposite and two maxima M2 diametrically opposite and placed at 90xc2x0 from the maxima M1.
Such an insert has the advantage of producing on a run-flat condition a vibrating warning excitation distributed mainly over harmonics 2 and 4 of the turn of the wheel, while avoiding the frequency of the turn of the wheel. That facilitates detection of those signals directly by the driver, because the excitation of two harmonics, instead of only one, increases the range of speeds in which the vibrations are perceptible. Another advantage is to avoid the confusion always possible with, notably, the vibrations of the engine speed when the insert produces mainly a single excitation frequency. Furthermore, when the vehicle is equipped with a detection device, as, for example, described in application WO 94/03338, the peaks of the vibrating warning signals are very easily perceptible with very few risks of false alarms.
According to another embodiment, the characteristic M presents as a function of azimuth xcex1 two maxima M1 diametrically opposite and three maxima M2 distributed over the circumference and offset from the maxima M1.
Such an insert produces on a run-flat condition a vibrating warning excitation distributed mainly over harmonics 2 and 3 of the turn of the wheel, as well as by combination on harmonic 5.
According to a third embodiment, the characteristic M presents as a function of azimuth xcex1 two maxima M1 diametrically opposite and four maxima M2 placed at 90xc2x0 from one another and offset by 45xc2x0 from the maxima M1.
This insert produces on a run-flat condition a vibrating warning excitation distributed according to harmonics 2, 4 and 8. This has the advantage of an easier detection at low speed.
Preferably, on both sides of each maximum M1 or M2, there is a single value minimum M3 that increases the concentration of energy actually included in the harmonics principally excited.
The alarm means can also contain, in combination with the first variation, a variation as a function of azimuth xcex1 of the second characteristic of the group of the outer radius and radial stiffness of said insert.
According to another embodiment of the invention, the means are a combination of a variation of the outer radius and a variation of the radial stiffness of the insert as a function of azimuth xcex1, so that the insert under load presents a crushed radius as a function of azimuth xcex1 containing at least four maxima.
The variations of radial stiffness as a function of azimuth xcex1 can range between 10 and 1000% and the variations of outer radius can range between 0.1 and 7 mm.
The variation of radial stiffness of the safety insert according to the invention preferably depends on the amplitude of the radial stress to which the support is subjected. For example, beyond a given radial stress threshold, the variation of radial stiffness as a function of azimuth xcex1 increases very markedly.
Such an insert can comprise a roughly cylindrical base intended to be fitted around the rim, a roughly cylindrical crown intended to come in contact with the tire in case of pressure loss and walls appreciably connecting the base and the crown radially. It can be made of flexible elastomeric materials and have arc-shaped walls. The variations of radial stiffness are then easily obtained by varying the thickness or shape of the walls and the variations of outer radius are preferably obtained by varying the lengths of the walls.
In one particular embodiment, the variations of radial rigidity and/or outer radius of the insert are applied axially only on a part of the insert intended to be placed toward the inside of the vehicle. That has the advantage of creating, on a run-flat condition, warning torques and, therefore, signals oriented both radially and transversely, which can facilitate their detection. Another advantage is that such inserts mainly transmit vibrating warning signals only on a straight run, while on a turn, the tires having a substantial camber, it is the xe2x80x9cinactivexe2x80x9d uniform part of the insert which mainly bears the load. The risks of disruption of vehicle behavior are thus reduced.
Finally, the variations of a radial rigidity and/or of outer radius of the insert can present a complementary axial development as a function of azimuth xcex1. Such a development has the advantage of creating a vibration complementing the previous ones, a component of which is oriented in the axial direction.